1. Field of the Invention
The present invention relates to a device for measuring the amount of a contaminant in a lubricant. The device according to the present invention is used, for example, to measure a concentration of carbon particles contained in a lubricant of an internal combustion engine of a vehicle, in particular, a diesel engine.
2. Description of the Related Art
Various contaminants such as fine sand particles contained in engine intake air and oxides produced by combustion of fuel and the like are introduced into an engine lubricant in accordance with an engine operating time. Particularly, in a diesel engine, it is known that a large number of carbon particles contained in the exhaust gas are introduced into and contaminate the lubricant. Since the carbon particles introduced into the lubricant accelerate wear of respective sliding portions of the engine, when the concentration of the carbon particles increases beyond a certain limit, the lubricant must be replaced.
A method for measuring the concentration of the carbon particles is known wherein a portion of the lubricant is sampled from the engine and passed through a filter, and the weight of particles collected on the filter is measured or the collected particles are centrifuged and weighed. However, with this method, the concentration of the carbon particles cannot be measured when the engine is driven, and any changes over a period of time cannot be measured.
Another conventional lubricant contamination measuring device disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 57-98842, is provided with a light source and a photo cell at two end portions of a lubricant reservoir so that the amount of light received by the photo cell is decreased in accordance with the amount of contaminant in the lubricant.
In such a device, when the distance between the light source and the light receiving element is relatively far apart, the concentration of the carbon particles in the lubricant cannot be measured, for the following reasons. A decrease in a light transmittance due to the introduction of carbon particles in the lubricant can be expressed in the following equation in accordance with various experiments conducted. That is, when a light transmittance of the contaminated lubricant with respect to a noncontaminated lubricant is given by T, a distance between the light source and the light receiving element, more specifically, an optical path length in the lubricant, is given by D (mm), and a coefficient is given by K, a carbon weight concentration .alpha. (%) can be expressed by .alpha.=1/(K.D).log(1/T). The coefficient K is about 17.4 (1/mm). From the above equation, the light transmittance T with respect to the carbon weight concentration is expressed by T=10.sup.-K.D..alpha..
The light transmittance with respect to the carbon concentration changes significantly in accordance with a change in the optical path length D. When the optical path length D is 1 mm, the concentration .alpha. is about 0.2%, and the light can hardly pass through the lubricant, more specifically, T=3.3.times.10.sup.-4. The transmitted light is converted into an electric signal by the light receiving element opposing the light source to drive an indicator such as a concentration indicator. However, in a general electric signal processing circuit, the lower limit of a signal magnitude ratio for processing signals is about 1:1000. Therefore, as described above, when the optical path length D=1 mm, since the transmittance T of the light reaches 0.001 at the carbon concentration of 0.172%, the signal becomes too weak in practice when the concentration exceeds this value, and therefore, a concentration measurement cannot be performed.
When the optical path length is set to be short, a transmittance curve is approximately a straight line, and the range in which a concentration measurement can be performed can be widened. Although the required range of concentration measurement differs in accordance with the type of engine, an upper limit of the concentration .alpha. is generally about 0.5% to 4%. For example, when a concentration .alpha. of up to 0.5% is to be measured, the optical path length D satisfying T=0.001 for .alpha.=0.5% is 0.034 mm. Similarly, when a concentration .alpha. of up to 4% is to be measured, the optical path length D satisfying the same condition becomes 0.043 mm. As described above, when the carbon concentration in the lubricant is measured using light transmittance, the optical path length is an important factor. In practice, unless the optical path length is 0.34 mm or less, the necessary range of concentration cannot be measured. Thus, the optical path length must be set to be 0.34 mm or less.
To widen the range of concentration measurement, as described above, the optical path length D must be made very much shorter. However, in this case, the following problems occur. First, a variation in the measurement due to the optical path length occurs. This is because when an optical path gap (defining the optical path length) changes slightly by, e.g., several tens of micrometers, the relationship between the carbon concentration and the light transmittance is widely varied. That is, the optical path gap must be precisely defined. Second, when the optical path gap is shortened as described above, the rate of replacement of the lubricant between the light source and the light receiving element is low. For example, when the contaminated lubricant is replaced with new lubricant, a new concentration value cannot be quickly indicated. Furthermore, agglomerated contaminant particles can easily clog the optical path gap.